1. Field of the Invention
The present invention relates to a process for producing microcapsules using amino resin, such as melamine/aldehyde polycondensate and urea/aldehyde polycondensate, as a capsule wall.
2. Prior Art
Microcapsules have been used for changing or controlling various properties of substances in such fields as pharmaceuticals, agricultural chemicals, perfumes, and dyestuffs. The encapsulated methods can be divided into mechanical method, physical method, physicochemical method, chemical method, and methods based upon combinations of these. A variety of properties are exhibited by the capsules prepared by these methods, then they are suitably selected depending upon the core material that is to be encapsulated and its purpose of use. With the mechanical and physical methods, in general, the capsule particles are large, and the impermeability of the wall films is not very high. With the physicochemical and chemical methods, on the other hand, the capsules have small particle size (on the order of microns) which are ideally distributed, and the wall films have a high impermeability.
Microcapsules are principally used for the production of carbonless papers. The microcapsules for carbonless papers are manufactured by the physicochemical or the chemical method.
The physicochemical method can be represented by a coacervation method which usually uses gelatin. This method is capable of encapsulating a wide range of hydrophobic materials, and has been widely employed since it was announced in U.S. Pat. No. 2,800,457 in 2957 . The coacervation method utilizes natural materials. With this method, therefore, the quality varies, the capsules have poor water resistance, the concentration of the capsule slurry is not high, and the wall film impermeability is not satisfactory.
With the chemical method, the resin for forming capsule walls is obtained from monomers or oligomers. The chemical method can be divided into an interfacial polymerization method in which the reaction starts from both the core material and the continuous phase, and an in situ polymerization method in which the reaction starts from either the core material or the continuous phase. With these methods, as a wall material, the monomers or oligomers can be selected from a wide range, and as a core materials, from hydrophobic materials through up to hydrophilic materials can be encapsulated. Generally, however, the starting materials have such strong reactivity that they react with the core material and many of the starting materials are toxic. Further, with these methods it is difficult to form thick wall films. Accordingly, the chemical methods have been employed only for particular uses.
The present invention relates to an encapsulating process which pertains to the in situ polymerization method. More specifically, the present invention deals with a process for preparing microcapsules having a wall film composed of a polycondensate of melamine/aldehyde or urea/aldehyde in a system in which a hydrophobic core material is dispersed or emulsified in an acidic aqueous solution which contains an anionic colloidal material. This anionic colloidal material is an anionic, water-soluble copolymer consisting of three or more monomers which include at least acrylic acid, hydroxyalkyl acrylate or hydroxyalkyl methacrylate, and styrenesulfonic acid.
Use of melamine/formaldehyde polycondensate or urea/formaldehyde polycondensate for forming the wall film has already been disclosed in, for example, Japanese Patent publication Nos. 12380/1962, 3495/1969, and 23165/1972. With these methods, it is difficult to efficiently and stably deposit the polycondensate around the hydrophobic core material, or the emulsification or dispersion is not efficiently performed.
In order to improve the above-mentioned defects, Japanese Patent Publication No. 16949/1979 proposes to use, as the anionic high-molecular material, a polyethylene/maleic anhydride, polyacrylic acid, polymethyl vinyl ether/maleic anhydride, and the like. This method helps improve the emulsification and dispersion properties and the depositing efficiency of the polycondensate around the core material. When the polyethylene/maleic anhydride or the polymethyl vinyl ether/maleic anhydride is employed in this method, however, high temperatures and extended periods of time are required before the copolymer dissolves, and the resulting capsule slurry exhibits an undesirably high viscosity.
Further, Japanese Patent Laid-Open No. 47139/80 teaches to use a copolymer of styrene/maleic anhydride and a copolymer of vinyl acetate/maleic anhydride in combination, to improve the emulsification and dispersion properties, in order to obtain a stable capsule slurry having low viscosity. However, since the copolymer of styrene/maleic anhydride precipitates when the pH is smaller than 4, it cannot use the method which uses the urea/formaldehyde as a capsule wall-forming agent that efficiently reacts at low pH values, or the method in which urea or ethyleneurea is added at a pH of less than 4 in order to remove the unreacted formaldehyde after the capsules have been formed.
According to Japanese Patent Laid-Open No. 51238/81 which produces capsules having a wall film composed of melamine/formaldehyde polycondensate, a polymer of the type of vinylbenzenesulfonic acid is used as the anionic high-molecular material, to obtain a high-concentration, low-viscosity capsule slurry which permits the unreacted formaldehyde to be processed at low pH values. When this method is adapted to the preparation of capsules having wall film of urea/formaldehyde resin, however, it is difficult to obtain a dense capsule wall, and the whole system often coagulates when the encapsulating conditions are changed even slightly. Further, a large quantity of foam is produced when the polymer dissolves which makes it difficult to prepare capsules maintaining good operability and stability.